In crop-raising agriculture, efforts have been made to reduce the amount of tillage of crop land through "no-till" and "reduced till" farming practices. These efforts have led to the developments of tillage implements such as chisel plows and anhydrous ammonia applicator knives, which typically employ a curved shank on which a soil-engaging tooth is mounted. The curved shank is pivotally mounted to a base plate which is mounted to a horizontal tool bar which is drawn by a tractor. The curved shank of the tillage implement is biased in position upon the base plate by a coil spring in compression which resists the rearward movement of the tillage implement as it is drawn through the soil. When the tillage implement encounters an obstacle such as a rock, the tillage implement is forced rearward and upward against the resistance of the coil spring. As the tillage implement overcomes the obstacle, the coil spring forces the tillage implement back into position in the soil. As the tillage implement rises from the ground, the coil spring resists with increasing force and when a sufficiently large obstacle is encountered, the tillage implement may be displaced upwardly to a point at which the coil spring is fully compressed and it binds. In this condition, the coil spring is no longer acting as a biasing member but becomes a stop and this leads to bending of the base plate or the shank, or breakage of the soil engaging tooth on the lower end of the curved shank.
Efforts to reduce the potential of the coils to fully compress, such as by lengthening the spring have not been successful because the longer the coil spring, the greater its tendency to deform laterally, rather than to uniformly compress. Efforts to substitute the compression spring with an extension coil spring have not been successful due to the large static force needed to hold the tooth below the surface of the ground to be tilled as the tool bar is drawn across the ground.
When tilling rocky ground, it is common for prior art tillage implements to become damaged including through bending of the base plate or by breakage of the tooth. Repair is costly and time consuming. There is a need to improve the structure of tillage implements to provide a constant resistive force as the displacement of the tillage implement increases and to allow a greater range of displacement of the tillage implement, including sufficient displacement for the tooth of the tillage implement to rise from a soil depth of six inches to a height completely out of the soil and even above an outcropping rock. While existing coil spring biased tillage implements may permit a vertical displacement of the tillage implement's tooth of four inches, a larger range of displacement is desirable.
Previous devices which employ hydraulic cylinders to bias a plowshare in the soil include the devices illustrated in U.S. Pat. No. 3,481,407. The device of that patent employed a hydraulic cylinder with its ram extended to bias a plowshare into soil contact. A system pressure of 2200 psi was maintained to bias the plowshare but when an obstacle in the soil was encountered, system pressure could rise to 2800 psi whereupon the relief valve pressure was overcome and fluid was allowed to escape from the cylinder. With high pressure hydraulic fluid passing by a relief valve, heat is generated and extended overheating of the hydraulic system will result in premature system failure. Seed drills having a hydraulic reset feature are disclosed in U.S. Pat. No. 4,353,423 and 4,422,511 wherein a hydraulic cylinder with its ram normally in an extended position biases a gang of discs against the soil surface. When a soil obstacle is encountered, the disk gang is displaced upwardly causing the ram to retract and force hydraulic fluid through a relief valve. An extended ram being forced into the cylinder provides a point of instability in the system.